The present invention relates to a new and improved construction of circuit for connecting a process computer by means of a number of analog channels with a machine tool.
Generally speaking, the circuit arrangement of the present development is of the type containing an interface at its input side, a digital-to-analog converter (D/A-converter) for converting digital data received from the process computer into analog signals which are to be delivered to analog channels. Additionally, there is provided a sequencer for multiplex control of the signal output to the analog channels, and there are also provided analog storages which are connected forwardly of the analog channels.
With a heretofore known circuit of this type, as disclosed for instance in the brochure "iSBX 328" of the Intel Corporation, dated September 1980, there are provided eight analog output channels at which there can be outputted an output voltage or potential of maximum .+-.5 volts (asymmetrical), and the resolution amounts to 12-bits. If for this resolution of 12-bits there is formed from the digital data furnished by the process computer a variable output voltage of .+-.5 volts, then this means that in the step curve of the analog voltage signal each step or stage represents a voltage surge of minimum 2.4 mV, because the bit with the lowest significance or place furnishes a voltage of 2.4 mV. Such voltage steps or stages in the analog output voltage signal are too large for regulation circuits provided for a machine tool, because they can excite the regulation circuits into oscillation. Such undesired excitations can only be avoided with a quasi-continuous signal, not however with a signal which possesses voltage steps of minimum 2.4 mV. The known circuitry is therefore also predominantly used for supplying analog voltages for measuring instruments, cathode ray tube controls, programmable current supply systems, and positioning or adjustment elements conventionally used in industrial applications.
For these fields of use there is adequate the resolution of 12-bits and the signal quality attainable with an asymmetrical output voltage of maximum .+-.5 volts (i.e. -5 to +5 or 0 to +5 volts). Since with the known circuit it is only possible to work with two bytes, i.e. 16-bits, because of the Intel interface component 8041A provided at the input side, and as to such 16-bits 12-bits thereof are intended for the data and the remaining 4-bits for the channel control and the like, the resolution also cannot be increased to a higher bit number with the same number of channels. Additionally, the asymmetrical output voltage of maximum .+-.5 volts, generated by the known circuitry, is too small for the functional reliability required for regulation circuits of machine tools, because machine tools usually work in rooms or areas containing pronounced spurious or disturbing effects and there can be interposed between the input interface of the circuit and the receivers, for instance positioning or adjustment elements, conductors or lines having lengths of 5 to 10 meters. The greater the voltage of the control signals that much smaller is, however, the influence of the spurious signals. A simple increase of the delivered voltage, for instance by providing an amplifier having a gain of 2, would increase the functional reliability and also would increase the aforementioned voltage steps in the analog output signal from a minimum of 2.4 mV to a minimum of 4.8 mV, however such would be even more unfavourable for regulation circuits, since there would then be present a greater danger of the excitation of oscillations. Also, the low number of eight output channels, with the heretofore known circuitry, could not be increased because the analog storage (sample and hold storage or sample storage) provided at the output of the analog output multiplexer contained in the circuit, would require a continuous refreshing of its storage content. With the known circuit there is refreshed a single channel with a frequency of 5 kHz, whereas all eight channels are refreshed with 1 kHz. If the number of channels were increased, then the refreshing frequency would be too low and there would exist the danger that the sample storage no longer could be periodically refreshed at the proper point in time. Furthermore, with the heretofore known construction of circuit what is disadvantageous is that there is not contained therein any galvanic decoupling. If, specifically, the known circuitry were connected with a machine tool, where for safety reasons there is always provided earthing, then the circuit, as concerns its potential, could not be operated independent of the machine tool, so that the functional reliability would be reduced. The total operating course or cycle, namely selection of each active channel, selection of the operating mode, data transmission and so forth, is only controlled by software with the known circuit, and which is contained in the input interface 8041A.